DRUMLIN STRUCTURE AND ORIGIN 705 



but when given that shape they resisted the ice impact better than harder 

 rocks, as the product of the ice rubbing was a lubricant and plastic paste, 

 essentially like clayey till in its mechanical properties. The shale hills were at 

 first compliant, and then resistant to the ice. They became drumlins in effect, 

 though not in origin, being erosional forms, not constructional. 



The shaping of hills of the softest rock instead of leveling them is an 

 evidence of erosional weakness of the ice in the drumlin belt. Vigorous abra- 

 sion of hard rocks would not be consistent with drumlins in the same locality. 



Relation to Moraines and to glacial Lakes 



During the stage of ice waning represented by the dominant drumlin area the 

 ice-front was swept by vigorous rivers on the higher ground and was faced by 

 lakes on the lowest ground. The drumlins reach up to the north side of the 

 drainage channels in good strength, but they fade out into attenuated forms in 

 the areas where the ice-front was not swept by streams, and where consequently 

 the drumlin tracts are fronted by moraines. These moraines represent only the 

 superglacial and higher englacial drift, carried to and dropped at the ex- 

 treme edge of the ice, while the drumlins were forming at the same time from 

 the subglacial and lower englacial drift beneath the ice-sheet, in the rear of 

 the moraine. 



Theoretically the moraines should be weak where the drift was largely left 

 in drumlin form, and the facts seem in accord. 



Formation; Mechanics 



The idea that drumlins represent overridden moraines, or are erosional in 

 origin, may be true of some drumlins, but certainly is not true of the majority 

 of New York drumlins, which were constructed or built up by a plastering-on 

 process. 



In the mechanics of drumlin construction three sets of factors are recog- 

 nized : (a) factors pertaining to the ice itself; (&) those relating to the druin- 

 lin-forming drift; (c) the external influences of topography and climate. 



The dynamic factors pertaining to the ice body (a) include: (1) vertical 

 pressure; (2) horizontal pressure; (3) vigor and velocity of flow; (4) differ- 

 ential flow; (5) plasticity. 



The factors relating to the drift (&) are: (1) volume of the drift; (2) posi- 

 tion of the drift; (3) quality of the drift. 



The factors of external control (c) are: (1) general land slope; (2) minor 

 features of the topography; (3) temperature and water supply. 



The building up of the drumlins is coincident with the rubbing off or shaping 

 effect. As masses, the hills were built by accretion of the drift, but the convex 

 forms are due to the erosional factor. The whole process may be compared to 

 the work of the sculptor on a clay model — a plastering on and rubbing away. 

 The accretion is due to the greater friction between clay and clay than between 

 clay and ice. The hills of accretionary drift resisted the ice impact and rasp- 

 ing effect in the same manner as did the hills of shale. The form possessed by 

 both classes of hills is that which opposed the greatest resistance to removal 

 by the ice or the least resistance to the overriding movement of the ice. 



The drumlins were shaped by the sliding movement of the lowest ice, that in 



